How do you discover unused GPIO on the hardware abstraction layer?

In have a router on OpenWRT with an undocumented button I want to find. It is a MT7621 in a BGA so I can’t trace the hardware and rework is no go. It has the breed bootloader but it’s all in Chinese. Llama2 70B says to use github.com/rust-embedded/gpio-utils but I don’t see how that can really detect a change state on an unknown pin. I’m curious what software tools work for this application. I have a stack of Linux books beside me and a beer, but figured it would be fun to ask here to see if anyone has recommendations too.